This invention relates generally to a computer terminal having a video display, and particularly relates to a computer terminal energized by signals from a video display incorporated therein.
A computer terminal generally includes a power supply, a video monitor, a keyboard and logic circuitry. The user provides inputs via the keyboard and receives visual feedback from the monitor. The power supply generally energizes all video terminal components and the digital logic circuitry permits the terminal to perform sophisticated computations and/or information retrieval operations and to display this information on the video monitor for the operator's use.
Prior art computer terminal power supplies are generally either of the linear regulator type or of the switch mode type. The switch mode type of power supply generally utilizes a single voltage regulator with various conductive modes selectively switched in to provide various DC output signals. The problem with this approach is that the switching of all of the non-linear regulators occurs during the video display time interval producing display distortions during switching. The linear regulator power supply typically generates many B+ voltages from a power transformer and provides linear regulation for each of the B+ voltages. This approach is expensive because of the potentially large number of regulators required and is inefficient from a power utilization standpoint. In addition, the large power transformer required generates a substantial amount of heat which must be eliminated and the inductive leakage of this large transformer can readily degrade video imagery. In addition, those prior art computer terminal power supplies which have provided adequate isolation between the line voltage source and the various computer terminal components energized therefrom, have done so at low signal frequencies and have suffered from such limitations as increased size and signal filtering requirements. It is particularly important in this environment to provide adequate isolation for computer terminal logic circuitry which is highly susceptible to damage or program dropouts caused by fluctuations in the utility-provided line voltage source.
FIG. 1 shows a typical prior art power supply arrangement for a computer terminal. A line voltage source 10 typically provides 110 VAC or 220 VAC to a computer terminal power supply 12. Power supply 12 then provides various DC voltage levels to computer logic system 14 which generates synchronization signals which are then provided to the computer terminal/display 16. These sync signals provide proper timing between the horizontal and vertical sweep of the electron beam in the video display's cathode ray tube and the rate at which video information is provided to the video display. In addition, power supply 12 simultaneously provides energizing voltages to the various deflection systems and video signal processing and amplification circuitry in the video display. In this configuration the line operated power supply 12 provides some level of isolation for computer terminal logic system 14 from the high voltage line source 10 but at low signal frequencies thus encountering operating problems associated therewith.
One approach to a video display power supply design is disclosed in U.S. Pat. No. 3,641,267 to Cavallari wherein is described a power supply designed for stepping down an AC or DC input source to a stable DC input level. In this power supply a chopper circuit incorporating one or more normally blocked transistors is inserted between the input circuit and a load circuit. Voltage stabilization of the output signal is achieved by incorporating a voltage-limiting device such as a Zener diode in the input circuit which limits the amplitude of the output of the input transistor and hence the voltage level of the power supply output signal. With a transformer incorporated in the network and the Zener diode coupled to a secondary winding of the transformer, the current flowing through the primary winding will be controlled by the Zener in that excess secondary current is dissipated through the Zener diode connected between the base and the emitter of the associated transistor. The Cavallari invention is intended primarily to permit a large number of stable DC level output signals to be produced from an AC or DC input signal varying in voltage. The requirement for an expensive step down voltage transformer is avoided in Cavallari by providing the unblocking pulses to the normally blocked transistors from the flyback pulse of the horizontal sweep circuit. Cavallari, as such, is not intended to provide the isolation from the input line voltage source required for the sensitive logic circuitry of a computer terminal.
The present invention is intended to avoid the aforementioned computer terminal and video display power supply problems by providing a plurality of direct voltage signals to logic circuitry in a computer terminal in which the logic circuitry is isolated from the input line voltage source and the various DC signals provided to the logic circuitry can be controlled by a line operated power supply which provides high voltage isolation for sensitive computer terminal components and systems.